1. Field of the Invention
This invention relates to an interferometer using a laser beam, and more particularly to an interferometer which is arranged to facilitate laser tube replacements.
2. Description of the Prior Art
Laser interferometers have been used for inspecting and measuring infinitesimally fine surface conditions such as flatness or sphericity of an object, and are generally arranged to measure the surface condition of an object or specimen under inspection by irradiating the object or specimen with a laser beam which is projected from a laser tube through a reference plate, producing an interference fringe by the interference between the reflected light from the object and the reflected light from the reference plate, and taking the image of the interference fringe by an interference fringe imaging means. The laser tube of this sort is usually accommodated in a housing along with an interference fringe image sensor means, an optical laser beam guide means, and an optical interference fringe imaging means. This housing is maintained in a shielded state to protect the housed components against dust.
The output power of the laser tube drops after operation over a certain period of time, so that it has to be replaced at a suitable point in time. The replacement of the laser tube requires not only mounting a fresh laser tube in place of the old one but also adjusting its optical axis relative to the optical laser beam guide means. This job is troublesome and takes a great deal of time. Besides, since the housing has to be held in an opened state while replacing the laser tube, there are possibilities of dust or foreign matter entering the housing and being deposited on optical members of the laser guide means, interference fringe imaging means or on other built-in components.
Further, the laser tube which is relatively elongated in shape gives rise to a problem in that the size of the housing has to be increased for accommodation a long laser tube. Besides, in order to support a housing of such a large size stably, it becomes necessary to mount the housing on at least four supporting posts, which, however, complicates the interferometer construction and impedes automation of the operations of loading and unloading specimens.